If this is reflecting onto a different channel (frequency) then it shouldnt interfere with the original signal (except perhaps by reducing its effective power output by a small amount).

It is a general FCC rule that radios and certain other types of devices are not supposed to intentionally cause interference, but in this case there shouldn't be any. Even if there was same channel interference, its not necessarily considered intentionally harmful - for example, its not illegal to use the same WiFi channel as your neighbor as long as you are within other specified technical limits such as transmit power or antenna gain. But doing so for the express purpose of interfering with them would be.